In an optical disk drive, a track-following servo system is used to make a light beam emitted by an optical head be projected accurately onto a track on an optical disk. As for detection of tracking errors, one-spot method and three-spot method are known.
The one-spot method employs an optical-head drive of a so-called swing-arm type which moves an optical head in an arc. This allows simplification in the configuration of an optical-head drive. With this method, however, it is difficult to achieve accurate tracking.
Meanwhile, with the three-spot method, as illustrated in FIG. 8, in addition to a main beam for detecting a signal, two sub-beams 2 and 3 are projected onto each side of the main beam on a track 1 of the optical disk. The optical head is driven so that the sub-beams 2 and 3 are equally projected onto the track 1. The main beam and sub-beams are produced by splitting a light beam into zeroth-order diffracted light, positive first-order diffracted light and negative first-order diffracted light by means of a diffraction grating.
When moving the optical head along a guide shaft from the inside (D portion shown in FIG. 8) toward the outside (E portion) of the optical disk, it is very important to manufacture the guide shaft with high-precision in order to move the optical head correctly in a radial direction (X direction) of the optical disk. Moreover, it is also necessary to fix both ends of the guide shaft to the chassis of an optical disk drive accurately. These requirements cause a drawback that the cost of manufacturing the optical-head drive increases.
Furthermore, the radius of curvature of the arc of the track 1 varies from the inside to the outside of the optical disk (as for the outside of the optical disk, it is assumed for the sake of explanation that the radius is infinite and the track 1 is illustrated as a straight line in the figure). Therefore, even when an angle .alpha. formed by a straight line M.sub.1 connecting the centers of two sub-beams 2 and 3 and a straight line M.sub.2 passing through the center O of the optical disk and the midpoint of the straight line M.sub.1 takes the same value in the inside and the outside, i.e., when the optical head is correctly moved in a radial direction of the optical disk, the area of a portion of the track 1 exposed to the sub-beam 2 varies from the area of a portion of the track 1 exposed to the sub-beam 3. This results in a decrease in the accuracy of tracking.
Namely, when the sub-beams 2 and 3 strike portions of the track 1 having the same area respectively in the inside, the area of a portion of the track 1 exposed to the sub-beam 2 is smaller than that exposed to the sub-beam 3 in the outside. Then, the track-following servo controls the sub-beams 2 and 3 so that portions of the track 1 having the same area are exposed to the sub-beams 2 and 3, respectively. In this case, the area of the portions of the outer track 1 exposed to the sub-beams 2 and 3 are smaller than that of the inner track 1 exposed to the sub-beams 2 and 3. This causes a lowering of the accuracy of the tracking.
Additionally, regarding a mechanism for moving the optical head along the guide shaft, for example, a mechanism using a feed screw and a rack-and-pinion mechanism are listed. Both of them, however, require high-precision components, and which contributes to an increase in the cost of manufacturing the optical-head drive.